Magnetron modulation system



Aug. 2, 1938. A. W.-LADNER MAGNETRON MODULATION SYSTEM 3 Sheets-Sheet 1 I Filed June 7, 1934 JOURCE 0F MODULATIONS iNVENTOR 44,4 #4 1404 52 fig ATTORNEY Aug. 2, 1938. A. WJLADNER 2,125,507

MAGNETRON MODULATION SYSTEM Filed June 7, 1934 3 Sheets-Sheet 3 I 0 l 0 i Sol/Rd? 0F MODULnT/ONS INVENTOR ATTORNEY Patented Aug. 2, 1938 PATENT OFFICE MAGNETRON MODULATION SYSTEM Alan Wilfrid Ladner, Goonhilly, Danbury, England, assignor to Radio Corporation of America, a corporation of Delaware Application June 7, 1934, Serial No. 729,382 In Great Britain June 28, 1933 5 Claims.

This invention relates to oscillation generating arrangements including magnetrons and has for its principal object to provide an improved arrangement whereby efficient and deep modulationof the oscillations generated by a magnetron may be obtained.

According to the main feature of this invention modulation of an oscillation generator comprising a magnetron is effected by simultaneously varying the magnetic field of said magnetron and also the anode potential thereof.

It is, of course, known per se to modulate a magnetron oscillation generator by varying the anode potential thereof and it is also known per se to modulate a magnetron oscillation generator by varying the magnetic field thereof but the use, according to this invention, of the two methods in combination avoids serious disad- Vv vantages which are present when either of the if) two methods known per se is employed separately.

In order that the advantages and nature of the invention may be the better understood, reference will be made in the following description to the accompanying drawings in which Figures 1, 2, 4, 5 and 6 show, with slight variations, typical circuit diagrams for a magnetron discharge tube in combination with modulating means having the novel features of the invention;

Fig. 3 shows an embodiment of means for producing a magnetic influence upon the electron discharge in the tube; and

Figs. 1a, 1b, 2a, 2b, 3a and 3b are characteristic performance curves for certain embodiments 5*; of a magnetron discharge tube and of my invention.

Consideration will first be given to the well known case of a magnetron having a split anode arrangement and in which modulation is effected by varying the magnetic field. The curves shown in Figs. 1a and 1b of the drawings are typical of the results obtained with such an arrangement. In Fig. 1a the curve I05 is a curve of high frequency current (ordinates) plotted against magnetizing current (abscissae), while the curve Ia is a curve of direct current feed (ordinates) plotted against magnetizing current (abscissae). In Fig. 1b the abscissae are again variations of 'magnetizing current, but the curve PI is a curve of power input, the broken line curve E is a curve of efliciency and the curve PO is a curve of power out-put. The curves of Figs. 1a. and. 1b were obtained with a magnetron oscillator operating on a wave length of three metres with a fixed anode voltage, modulation being effected by varying the magnetic field, i. e., the magnetizing current. From these curves the following results appear:

(1) Modulation of the magnetic field does not give linear modulation of the high frequency output current since the curve I05 is not a straight line and has no substantially rectilinear portion.

(2) There is a marked hysteresis effect when r the magnetic field is reduced to zero and then brought up again (this is indicated in the usual conventional manner in Fig. 1a by the dotted line curves and arrow heads).

(3) Although the efiiciency at one point of working is high it is at other points extremely low. Therefore, although modulation by varying the magnetizing current does cause a change in high frequency output, there is also a substantial change in efficiency and accordingly the dead loss through the magnetron is increased as the output falls. It follows, therefore, that modulation by this method can only be accomplished where the magnetron is operated in such circumstances that its average condition of working is greatly under its maximum output condition, and its average efficiency greatly less than its maximum efiiciency.

Figs. 2a and 2b of the drawings show corresponding curves for the case where modulation is effected by varying the anode voltage, the magnetizing current being maintained constant. In the curves of Figs. 2a and 2b the references Ios, Ia, E, PI and PO have the same meaning as in Figs. 1a and lb, but the abscissae in Figs. 2a and 2b are values of anode potential, the magnetizing current being kept constant for any particular curve. In Fig. 2a the full line curves are those obtained when the magnetizing current is set to the best value for a particular voltage and it will be seen that when this. is done the .curves are characterized by discontinuity when the anode voltage is reduced (by modulation) and by very small change of output when the anode voltage is increased (by modulation) although the input remains high. Thus the efiiciency is high near the point of discontinuity and falls rapidly as the anode voltage is raised, and it is only possible to obtain shallow modulation. The dotted curves of Fig. 2a are shown for the case where the magnetizing current is reduced to a lower value than the best value, resulting in a reduction of output and a lowering of efiiciency. As will be apparent from the curves, it is now possible with the lower magnetization, to obtain a greater range of output variation by varying the anode voltage but the efficiency is extremely low. The results represented graphically in Figs. 2a and 2b may be summarized by the statement that when anode voltage modulation alone is employed the choice is between shallow modulation at a somewhat low efiicency, or deep inodulaton at a very low efficiency, and a low average output.

Figs. 3a and 3b of the drawings show graphically the results obtained by employing, in accordance with this invention, both anode voltage modulation and magnetizing current modulation simultaneously. As will be .apparent from Figs. 3a and 3b the input and output vary together and therefore at all points the magnetron is operating at a relatively high efliciency and in consequence with low dead loss. It would further appear that the present invention enables very deep and linear modulation to be effected, probably up to 100%, although the obtaining rectilinearity of modulation at the top of the modulation curve will be dependent upon whether the filament of the magnetron is capable of giving the necessary emission.

The invention may be carried into practice in many different ways, some of which are illustrated in the accompanying drawings. For example, in one circuit arrangement in accordance with the invention and shown in Fig. l, a split anode magnetron i has two anode portions 2, 3

connected to the opposite ends of a tuned circuit 4, the center point 5 of which is connected through a choke 6 to the positive terminal I of a source (not shown) of anode potential having a negative terminal 8. Modulation by the well known choke method is employed, the anode cathode space of a modulating valve 9 to whose grid circuit the modulating potentials are applied at Hi being connected between the center point 5 of the tuned circuit 4 and the cathode ll of the magnetron,

' i. etc the terminal 8. The choke 6 may be arnecessary to provide a separate high inductance choke 6b in series with the magnetization winding 6a, the anode of the modulator valve 9 being connected to the junction of 6a and 6b, all as shown in Fig. 2.

In Figs. 1 to 6 inciusive, l2 represents the usual polarizing winding of the magnetron. In practice the iron core of the magnetizing winding (6 or 5a as the case may be) is arranged to influence the magnetron and a convenient practical design is shown as in Fig. 3, and consists of an iron core l3 having a rectangular window with a pair of pole pieces l5, l6 projectin from opposite sides of the window towards one another and separated by a relatively smail gap in which the magnetron lis inserted. On the pole pieces I5 and i6 is wound the polarizing winding l2 (if necessary) and the magnetizing win-ding 6 or Ea, the former coil (if provided) being, of course, energized to a desired degree from any convenient direct current source, e. g., the anode potential source at l, 8.

Ina modification shown in Fig. 4 and which has the advantage of improved flexibility of design. since'it does not require the winding of the magnetron field core to be such as to have to take a predetermined current value, a winding IS in the common part of theano-ole circuits of the magnetron and of the valve 9 constitutes the primary of a transformer whose secondary I4 is connected across the magnetizing winding 60 of the magnetron, the polarizing winding l2 being adjusted to set the field to the mean value.

The invention can also be carried into practice with the employment of series modulation. In one arrangement of this kind, shown in Fig. 5, the positive terminal 1 of the anode source is connected to a center tap 5 upon a tuned circuit 4 whose ends are connected each to one of the anode portions 2 or 3 of the split anode of the magnetron i, and the cathode H of the magnetron is connected through the magnetizing winding 6d of the said magnetron and thence through the anode cathode space of a modulating valve 9 to earth and terminal 8. In this embodiment a low inductance magnetizing winding would be employed.

In a slight modification (not illustrated) of the last mentioned arrangement, the magnetizing winding is .not connected directly in series with the modulating valve and the magnetron but is energized from the secondary of a transformer whose primary is connected in series with the said modulating valve and the said magnetron.

In a further modification shown Fig. 6 and in "which direct anode modulation is efiected, a center tapping 5 upon a tuned circuit 4 connected between the anodes 2, Sci a split anode magnetron i is connected through the secondary Be of a transformer to the positive terminal I of a source of anode potential and modulating potentials are applied at In to the primary E5 of this transformer. The secondary So of the transformer also constitutes or forms part of the magnetizing winding of the magnetron. In a slight modification of this last mentioned arrangement thesecondary of the transformer does not constitute part of the magnetizing winding of the magnetron but a separate magnetizing winding in series with the primary of the transformer is employed. f

Although in all the arrangements illustrated in the drawings a 'polarizingwinding' I2 is provided in order to enable the core of the magnetizing system of the magnetron to be set to its correct magnetic adjustment, this winding is theoretically not a necessity, though in practice it will almost always be required.

It will be appreciated that in order to obtain simultaneous modulation of anode voltage and controlling or determining the phase of the mag netization changes relative to the anode voltage changes. Any means known per se may be provided for this purpose, e. g., chokes or resistances in series or capacities in shunt may be included in the magnetization circuit or in the main'feed (anode) circuit, or both, as may be required.

What" is claimed isf 1. In a system of the class described, a magnetron oscillator device having split anodes and a cathode, means including a magnetizing coil for producing a magnetic fieldwithin which saiddevice is tooperate, a source of direct current potential for activating said magnetizing coil and alsothe electrodes of said magnetron, an impedance in series with and of greater inductive value than said; magnetizing coil, both the impedance and the magnetizing coil constituting part of an anode potential supply lead for said magnetron device, and an electron discharge device having an input circuit connected to a source of modulations and an output circuit connected between the cathode of said magnetron and a point in said anode potential supply lead intermediate said impedance and said magnetizing coil.

2. In a modulator system, a magnetron having split anodes and a cathode, an anode potential supply circuit including output leads for said magnetron, a transformer having its primary winding included in one of said output leads, a magnetizing coil within the magnetic field in which said magnetron is disposed, said coil being in circuit with the secondary winding of said transformer, and an electron discharge tube having its input circuit connected to a source of modulations and its output circuit in shunt between the cathode of said magnetron and a point on one of said anode potential supply leads intermediate said transformer and the anodes.

3. In a modulator system, a magnetron discharge tube having a cathode and divided anodes, a direct current potential source connected between said cathode and said anodes, a frequencydetermining circuit interconnecting said divided anodes, a magnetic field coil for controlling the lines of magnetic force within said tube, and a modulation circuit including an electron discharge tube having its space discharge path in a circuit between the cathode of said magnetron and a nodal point on said frequency-determining circuit, the last said circuit including also a portion at least of said magnetic field coil and constituting shunting means for varying the direct current anode potentials which are supplied to said magnetron discharge tube and also to said magnetic field coil.

4. In a modulator system, a magnetron discharge tube having a cathode and segmented anodes, a frequency determining circuit interconnecting said segmented anodes, a direct current potential source connected between said cathode and a point on said frequency determin ing circuit, a magnetic field coil for controlling the lines of magnetic force within said tube, and a modulation circuit including an electron discharge tube having its space discharge path in shunt with the circuit which includes said magnetron discharge tube and said frequency determining circuit, said modulation circuit being arranged to simultaneously vary the potentials applied respectively to the electrodes of said magnetron discharge tube and to said magnetic field coil.

5. In a modulator system, a magnetron discharge tube having a cathode and divided anodes, a direct current potential source connected between said cathode and said anodes, a frequencydetermining circuit interconnecting said divided anodes, a magnetic field coil for controlling the lines of magnetic force within said tube, and a modulation circuit including an electron discharge tube having an input circuit under control of a source of modulating energy and a space current path constituting shunting means for varying the direct current potentials which are supplied to said magnetron discharge tube anodes through said frequency-determining circuit, and to said field coil.

ALAN WILFRID LADNER. 

